Today most organic chemicals are being produced by catalytic transformations of fossil resources such as oil, coal and natural gas. Within a few decades, the availability of these fossil resources is projected to decrease thus making it imperative to use alternative carbonaceous resources as feedstock. Carbohydrates are abundant and inexpensive carbonaceous resources available in nature. Since carbohydrates constitute a renewable and carbon neutral resource, it has become increasingly important to find feasible ways to convert them into useful chemicals such as 5-hydroxymethylfurfural (HMF), lactic acid, levulinic acid, and the like.
Levulinic acid has in particular been recognized as an important bio-derived platform chemical that may provide a source to produce chemicals and fuels (U.S. Pat. No. 5,859,263). Levulinic acid is also useful as a solvent, food flavoring agent, plasticizer, resin intermediate and building block for, e.g. tetrahydrofuran and succinic acid (C. K. Shu, B. M. Lawrence, J. Agric. Food Chem. 1995, 43, 782-784).
To produce levulinic acid, carbohydrates are traditionally being treated with aqueous mineral acid (H2SO4 and HCl) at atmospheric pressure at 100° C. (V. Sunjik, J. Horvat, B. Klaic, Kem. Ind. 1984, 33, 599-606). This method usually yields about 40% of levulinic acid. The yield of levulinic acid may further be improved to 60-70% by continuous flow conditions at higher temperatures and pressures using H2SO4 as catalyst (WO 89/10362 and WO 96/40609). S. Saravanamurugan, O. N. Van Buu, A. Riisager, ChemSusChem 2011, 4, 723-726, disclose conversion of mono- an disaccharides to ethyl levulinate with sulfonic acid-functionalized ionic liquids. However, a major drawback in these processes is tedious work-up during the separation stages. S. Saravanamurugan, A. Riisager, Catal. Commun. 2012, 17, 71-75 also disclose conversion of fructose to ethyl levulinate with sulfonic acid-functionalized SBA-15 catalysts.
Acidic ion-exchange resins have also previously been used as catalysts for the transformation of sucrose to levulinic acid (R. A. Schraufnagel, H. F. Rase, Ind. Eng. Prod. Res. Dev. 1975, 14, 40-44). Major disadvantages for these systems are moderate yields of levulinic acid of about 25% and maximum operation temperature around 150° C. due to thermal instability of the resin catalyst.
K. Lourvanij, G. L. Rorrer, J. Chem. Technol. Biotechnol. 1997, 69, 35-44 found Y-type zeolites to give moderate yields of levulinic acid (and minor amounts of HMF) when investigated as catalysts for the dehydration of fructose at temperatures between 110 and 160° C. In contrast K. Lourvanij, G. L. Rorrer, Appl. Catal. A 1994, 109, 147-165, found that a Fe-pillared montmorillonite catalyst was very active and able to convert glucose quantitatively, though with low selectivity of 20% to levulinic acid. Instead a much higher amount of formic acid as well as a significant amount of coke was observed in this study.
Zeolites are tridimensional crystalline aluminosilicates with the following formula in the as-synthesized form: xM2/nO.xAl2O3.ySiO2.WH2O where M is a cation which can belong to the group IA or IIA or can be an organic cation, while n is the cation valence, and W represents water contained in the zeolite voids. Crystalline structures of the zeolite type but containing tetrahedrally coordinated Si, Al, P, as well as transition metals and many group elements with the valence ranging from I to V such as, B, Ga, Fe, Cr, Ti, V, Mn, Co, Zn, Cu, etc., have been synthesized with the generic name of zeotypes, including AIPO4, SAPO, MeAPO, and MeAPSO type molecular sieves.
The main characteristic of the zeolites and zeotypes is that the tetrahedral primary building blocks are linked through oxygen producing a three-dimensional network containing channels and cavities of molecular dimensions.
Considering the channel size they are conventionally defined as ultralarge (>12-membered rings) with a free diameter above 8 Å, large (12-membered rings) with a free diameter of 6-8 Å, medium (10-membered rings) with a free diameter of 4.5-6 Å, or small (8-membered rings) with a diameter of 3-4.5 Å, pore materials depending on the smallest number of O, Al and Si atoms that limits the pore aperture of their largest channel. Examples of zeolites and zeotypes with different pore size are given in Table 1. The system of channels of these molecular sieves produces solids with very high surface area and pore volume, which are capable of adsorbing great amounts of substrate/reactants. This fact combined with the possibility to generate active sites inside of the channels and cavities of zeolites and zeotypes produces a very unique type of catalyst, which by itself can be considered as a catalytic microreactor.
In a catalytic reaction the reactant follows a sequence of events before it becomes a desorbed product. In the case of a zeolite, the sequence is diffusion of reactant through the zeolite micropores to reach an active site, adsorption of reactant on the active site, chemical reaction to give the adsorbed product, desorption of the product, and, finally, diffusion of the product through the zeolite channels. In the case of carbohydrate reactions on zeolites and zeotypes, where the size of a molecule closely matches the pore size, it is reasonable to think that the first step in the catalytic process, i.e. the diffusion of the reactant, can play an important role in the overall rate of the reaction observed. The configurationally diffusion is strongly dependent on the site and nature of the reactant, intermediates, product, the type of catalyst, and temperature.
TABLE 1Zeolites and Zeotypes and Their Ring Size for the Major Channel (taken from Chemical Reviews, 1995, 95, 559-614)Framework struct.typeCatalyst type(IUPAC CODE)type speciesring membersultralarge poreCLOcloverite20JDF-2020UFIVPI-5, MCM-9, AIPO4-5418AETAIPO4-8, MCM-3714large poreFAU,cubic and hexagonal12EMZfaujasite, SAPO-37BEAbeta12MORmordenite12OFFoffretite12MAZmazzite, omega, ZSM-412LTLLinde Tpe L12MTWZSM-12, MCM-2212SSZ-26, SSZ-23AFIAIPO4-5, SAPO4-512ATOAIPO4-31, SAPO-3112AFRSAPO-4012AFSMAPSO-4612AFYCOAPO-5012ATSMAPO-3612medium poreMFIZSM-5, silicate10MELZSM-1110FERFerrierite10ZSM-4810MTTZSM-2310TONZSM-222, theta I10HEUclinoptilolite10AELAIPO4, SAPO-1110AFOAIPO4-4110small poreLTAA8ERIerionite, AIPO4-178CHAchabazite8KFIZK-58RHORhO, BeAsPO-RHO8AEIAIPO48AFTAIPO4-528ANAAIPO4-248APCAIPO4-C, AIPO4-H3, MCM-18APDAIPO4-D8ATTAIPO4-33,8AIPO4-12-TAMUATVAIPO4-258AWWAIPO4-228AIPO4-128AIPO4-148AIPO4-14A8AIPO4-158AIPO4-218ATNMAPO-398CHASAPO-34, COAPO-44, 8COAPO-47, ZYT-6GISMAPSO-43LTASAPO-42
The flexibility in changing the adsorption characteristics of zeolites will allow discrimination between competing reactants, intermediates and products by modifying their relative adsorption interaction. Indeed, in the case of zeolites there are structures with low framework Si/AI ratios, and therefore with a large number of compensating cations which will produce very high electrostatic fields and field gradients in the channels and cavities. On the other hand, samples can be synthesized with high framework Si/AI ratios in which mainly dispersion forces are present, while very little or no influence from electrostatic fields and polarization forces will exist. In other words one could prepare zeolites with a very strong hydrophilic character which would preferentially sorb polar molecules or, on the opposite, with strong hydrophobic properties. In this way, one can change not only the total sorption capacity but also the relative adsorption, within the pores of the zeolite, of molecules with different polarity. This can be achieved by changing the framework Si/AI ratio by either synthesis or post synthesis treatments.
W. E. Farneth, R. J. Gorte have in Chem. Rev., 1995, 95, 615-635, discussed methods of characterizing the acidity of zeolites. The effect of both Brønsted and Lewis sites in a solid acid catalyst, such as zeolites, play an important role, but they are difficult to separate. The Brønsted effect relates to sites with a tendency to give up a proton, while the Lewis effect relates to sites with an electron-accepting property. However, zeolites are not molecules with a single type of acidic proton delivering and/or electron accepting feature, but rather collections of proton donor and/or electron accepting sites within a continuous framework. There may be a range of proton/electron affinities for a given zeolite. Different zeolites show very different specific rates of reactions, for example highly known in the field of hydrogen cracking. For low-silica faujasites, like HY, the catalytic activity increases as Al is removed from the lattice. In high-silica (Si/Al>10) materials like HZSM-5 and faujasites, however, catalytic activities increase linearly with Al-content for a number of reactions, particular in cracking.
S. Saravanamurugan, A. Riisager, in Catal. Commun. 2012, 17, 71-75 have shown that sulfonic acid functionalized SBA-15 is efficient and sulfonated zirconium less efficient in a catalyzed formation of ethyl levulinate from biomass-derived fructose and glucose. The non-functionalized zeolites ZSM-5, Y, beta and mordenite resulted in very little or no formation of ethyl levulinate. The experiments were performed in an ace pressure tube without pressurizing the reaction mixture contrary to the present method (Table 10). Compared to the zeolites of which some are available in nature in their pristine forms, the sulfonic acid functionalized materials are unnatural materials which are cumbersome to prepare and prone to loss of sulfonic acid functionality upon thermal treatment and recycling.